Motor control current sensor loss of assist mitigation for electric power steering

ABSTRACT

A power steering system includes a torque modifier module that generates a modified torque command in response to a current sensor fault, a magnitude of the modified torque command changes over a time period. The power steering system also includes a feedforward selection module that applies a dynamic feedforward compensation to a motor current command, thereby generating a motor voltage that is applied to a motor of the power steering system, the dynamic feedforward compensation modifies a frequency response of the power steering system.

CROSS-REFERENCE TO RELATED APPLICATION

This patent application claims priority to U.S. Provisional PatentApplication Ser. No. 62/109,698, filed Jan. 30, 2015, which isincorporated herein by reference in its entirety.

BACKGROUND OF THE INVENTION

The invention relates to motor control current sensor loss of assistmitigation for electric power steering (EPS).

EPS systems require the electric motor used to provide steering assistto be operated using a method of torque control. When using a PermanentMagnet Synchronous Machine (PMSM), Field Oriented Control (FOC) isutilized to allow the alternating current (AC) three-phase motor voltageand current signals to be transformed into a synchronously rotatingreference frame, commonly referred to as the d/q axis reference frame.In a d/q axis reference frame, the motor voltages and currents becomedirect current (DC) quantities. The FOC torque control technique iscommonly implemented either using feedforward methods of control or aclosed loop current feedback control.

When a closed loop current feedback control is used, the ability of thesystem to regulate the torque is heavily dependent on the measuredcurrents. However, current sensors, just like all sensors, are prone tofailures. The most common forms of errors in current measurement aregain and offset errors. Offset errors can be particularly problematic,because depending on the magnitude of the error, the torque ripplecaused by the offset error may become large enough to exceedrequirements related to maximum steering effort.

A common method for mitigating loss of steering assist due to a currentmeasurement fault is to transition from torque control utilizing acurrent regulator to achieve the desired motor current (and thus motortorque), to a torque control utilizing a static feedforward (inversemotor model) compensation when the fault is detected. However, afeedforward inverse motor model based torque control typically has muchlower bandwidth as compared to a high bandwidth current control loop.The motor torque control loop in an electric power steering system isthe actuator for the steering system, therefore should have a bandwidthseveral times higher than the outer steering assist control loop. Thestability compensation for the steering assist control loop is designedin a manner suitable for the higher bandwidth of the torque control whenthe closed loop current control is active.

A stability compensation designed for the lower bandwidth feedforwardinverse motor model based torque control used during a current sensorfault condition would be significantly different than the base stabilitycompensation. This produces the undesirable result during a currentsensor fault condition of the overall steering assist control loop beingless stable in the faulted condition than in the nominal unfaultedcondition.

SUMMARY OF THE INVENTION

In accordance with one embodiment, a power steering system comprises atorque modifier module that generates a modified torque command inresponse to a current sensor fault, a magnitude of the modified torquecommand changes over a time period, and a feedforward selection modulethat applies a dynamic feedforward compensation to a motor currentcommand, thereby generating a motor voltage that is applied to a motorof the power steering system, the dynamic feedforward compensationmodifies a frequency response of the power steering system, the motorcurrent command is based on the modified torque command.

In accordance with another embodiment, a power steering system comprisesa stability compensator selector module that selects a stabilitycompensator of a steering torque control loop of the power steeringsystem when a current sensor fault is detected, the stabilitycompensator generates a compensated torque command, and a torquemodifier module that generates a modified torque command from thecompensated torque command in response to a current sensor fault, amagnitude of the modified torque command changes over a time period, amotor voltage that is applied to a motor of the power steering system isbased on the modified torque command.

In accordance with another embodiment, a method for controlling a powersteering system comprises generating a modified torque command inresponse to a current sensor fault, a magnitude of the modified torquecommand changes over a time period; and applying a dynamic feedforwardcompensation to a motor current command, thereby generating a motorvoltage that is applied to a motor of the power steering system, thedynamic feedforward compensation modifies a frequency response of thepower steering system, the motor current command is based on themodified torque command.

These and other advantages and features will become more apparent fromthe following description taken in conjunction with the drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

The subject matter which is regarded as the invention is particularlypointed out and distinctly claimed in the claims at the conclusion ofthe specification. The foregoing and other features, and advantages ofthe invention are apparent from the following detailed description takenin conjunction with the accompanying drawings in which:

FIG. 1 illustrates a steering control system in accordance with someembodiments;

FIG. 2 illustrates a block diagram of a current regulator configurationin accordance with some embodiments;

FIG. 3 illustrates a block diagram of the motor electrical system inaccordance with some embodiments;

FIG. 4 illustrates a typical current sensor fault loss of assistmitigation algorithm in accordance with some embodiments;

FIG. 5 illustrates a plot of a torque command change during a currentsensor failure in accordance with some embodiments;

FIG. 6 illustrates a second plot of a torque command change during acurrent sensor failure in accordance with some embodiments;

FIG. 7 illustrates an open loop current control block diagram withstatic feedforward compensation in accordance with some embodiments;

FIG. 8 illustrates a comparison plot of the frequency responses of theq-axis direct transfer functions at an operating speed in accordancewith some embodiments;

FIG. 9 illustrates a loss of assist mitigation algorithm block diagramin accordance with some embodiments;

FIG. 9A illustrates a loss of assist mitigation algorithm block diagramin accordance with some embodiments;

FIG. 10 illustrates a loss of assist mitigation algorithm block diagramin accordance with some embodiments; and

FIG. 11 illustrates a block diagram for the motor control current loopunder a fault condition with dynamic feedforward compensation.

DETAILED DESCRIPTION

Referring now to the Figures, where the invention will be described withreference to specific embodiments, without limiting same, FIG. 1illustrates a steering control system 10. In the embodiment as shown,the steering control system 10 includes a steering control module 12, acurrent reference generator 14, a current loop compensator 16, a motor18 represented by the PMSM motor electrical plant, and a steering systemmechanical plant 20. The current loop compensator 16 may include acurrent regulator 24 along with a static feedforward compensator 22. Theoutputs of the current regulator 24 and the static feedforwardcompensator 22 are joined at summation block 26, to form a controlsignal for the PMSM motor electrical plant. The static feedforwardcompensator 22 may be active regardless of the feedback provided by theelectrical plant. Although a coupled P.I. configuration is shown in FIG.1, the subject matter disclosed herein is not limited to thisconfiguration.

FIG. 2 illustrates one type of current regulator 200 in accordance withsome embodiments. As shown, the control configuration includes severalsub-modules—a BEMF compensation module G_(F) 202, an integration module204, proportional compensation module C_(P) 206 and integralcompensation module C₁ 208, feedforward compensation module G 210, asubtraction module 212, and addition modules 213, 214. FIG. 2 alsoillustrates the motor 18.

The compensation modules G_(F) 202, C_(P) 206 and C₁ 208, and the plantP(s) of the motor 18 are 2×2 matrices. Signals I_(R), I_(E), I_(P),I_(A), I_(M), V_(P), V_(I), V_(C), V_(FF), V_(F), V_(R), V_(M) arevectors with two values each, corresponding to the d and q axes.

The current mode control configuration implemented in FIG. 2 may berepresented by matrix compensators. The following equations defined inthe d/q axis coordinate frame describe the plant transfer function(using line to neutral definitions):

$\begin{matrix}{V_{d} = {{L_{d}\frac{I_{d}}{t}} + {RI}_{d} + {\frac{N_{p}}{2}\omega_{m}L_{q}I_{q}}}} & \left( {{Equation}\mspace{14mu} 1} \right) \\{V_{q} = {{L_{q}\frac{I_{q}}{t}} + {RI}_{q} - {\frac{N_{p}}{2}\omega_{m}L_{d}I_{d}} + {K_{e}\omega_{m}}}} & \left( {{Equation}\mspace{14mu} 2} \right) \\{T_{e} = {{\frac{3}{2}K_{e}I_{q}} + {\frac{3}{4}{N_{p}\left( {L_{q} - L_{d}} \right)}I_{d}I_{q}}}} & \left( {{Equation}\mspace{14mu} 3} \right)\end{matrix}$

V_(d), V_(q) are the d/q motor voltages (in Volts), I_(d), I_(q) are thed/q motor currents (in Amperes), L_(d), L_(q) are the d/q axis motorinductances (in Henries), R is the motor circuit (motor plus controller)resistance (in Ohms), K_(e) is the motor BEMF coefficient (inVolts/rad/s), ω_(m) is the mechanical motor velocity in (in rad/s), andT_(e) is the electromagnetic motor torque (in Nm).

The torque equation may be nonlinear and may represent a sum of thetorque developed by leveraging the magnetic field from the permanentmagnets, and the reluctance torque generated by rotor saliency(difference between L_(d) and L_(q)) and predefined values of I_(q) andI_(d).

Equations 1 and 2 may be rewritten as follows:

V _(d) =L _(d) İ _(d) +RI _(d) +ωW _(e) L _(q) I _(q)  (Equation 4)

V′ _(q) =V _(q) −K _(e)ω_(m) =L _(q) İ _(q) +RI _(q) −ωw _(e) L _(d) I_(d)  (Equation 5)

In the above equations,

$\omega_{e} = {\frac{N_{P}}{2}\omega_{m}}$

is the electrical speed of the machine. To employ standard linearfeedback control design techniques, the machine speed is assumed to be aslowly varying parameter. It can be appreciated that due to relativelyslow flux dynamics, the quasi-static back-EMF (BEMF) term K_(e)ω_(m) canbe considered to be essentially constant, which is compensated as adisturbance in the feedforward path. These two assumptions allowlinearization of equations 4 and 5 for a fixed speed. Note that theapostrophe in the V′_(q) term is dropped in the equations below.

Equations 4 and 5 can re-written using s-domain representation asfollows:

$\begin{matrix}{U = {{P_{i}(s)}X}} & \left( {{Equation}\mspace{14mu} 6} \right) \\{\begin{bmatrix}V_{d} \\V_{q}\end{bmatrix} = {\begin{bmatrix}{{L_{d}s} + R} & {\omega_{e}L_{q}} \\{{- \omega_{e}}L_{d}} & {{L_{q}s} + R}\end{bmatrix}\begin{bmatrix}I_{d} \\I_{q}\end{bmatrix}}} & \left( {{Equation}\mspace{14mu} 7} \right)\end{matrix}$

Note that this description translates plant outputs into inputs via thecomplex frequency transfer matrix P_(i)(s), and is thus the inverse ofthe true plant transfer matrix. The block diagram for the abovedescription (with the additional BEMF term also shown) is shown in theblock diagram of the motor shown in FIG. 3. Specifically, FIG. 3illustrates the quasi-static back-EMF (BEMF) term K_(e)ω_(m) and themotor 18 includes matrix represented by equation 7.

The closed loop transfer matrix T relating the reference currents I_(R)to the actual currents I_(A) for the current control system shown inFIG. 3 may be written in terms of the matrix compensators as:

I _(A) =TI _(R)=(P ⁻¹ C)⁻¹(G+C)I _(R)  (Equation 8)

By inserting the appropriate compensator matrices in the aboveexpressions, the transfer matrix T may be expressed in equation 9 asfollows:

$T = {\begin{bmatrix}{T_{dd}(s)} & {T_{dq}(s)} \\{T_{qd}(s)} & {T_{qq}(s)}\end{bmatrix} = {\quad\begin{bmatrix}\frac{{\left( {{L_{q}s^{2}} + {\left( {R + K_{pq}} \right)s} + K_{iq}} \right)\left( {{\left( {\overset{\sim}{R} + K_{pd}} \right)s} + K_{id}} \right)} + {s^{2}{\overset{\sim}{\omega}}_{e}^{2}{\overset{\sim}{L}}_{d}{\overset{\sim}{L}}_{q}}}{{\left( {{L_{q}s^{2}} + {\left( {R + K_{pq}} \right)s} + K_{iq}} \right)\left( {{L_{d}s^{2}} + {\left( {R + K_{pd}} \right)s} + K_{id}} \right)} + {s^{2}\omega_{e}^{2}L_{d}L_{q}}} & \frac{{\left( {{L_{q}s^{2}} + {\left( {R + K_{pq}} \right)s} + K_{iq}} \right){\overset{\sim}{\omega}}_{e}{\overset{\sim}{L}}_{q}} - {\left( {{\left( {\overset{\sim}{R} + K_{pd}} \right)s} + K_{id}} \right)\omega_{e}L_{q}}}{{\left( {{L_{q}s^{2}} + {\left( {R + K_{pq}} \right)s} + K_{iq}} \right)\left( {{L_{d}s^{2}} + {\left( {R + K_{pd}} \right)s} + K_{id}} \right)} + {s^{2}\omega_{e}^{2}L_{d}L_{q}}} \\\frac{{\left( {{\left( {\overset{\sim}{R} + K_{pd}} \right)s} + K_{id}} \right)\omega_{e}L_{d}} - {\left( {{L_{d}s^{2}} + {\left( {R + K_{pq}} \right)s} + K_{id}} \right){\overset{\sim}{\omega}}_{e}{\overset{\sim}{L}}_{d}}}{{\left( {{L_{q}s^{2}} + {\left( {R + K_{pq}} \right)s} + K_{iq}} \right)\left( {{L_{d}s^{2}} + {\left( {R + K_{pd}} \right)s} + K_{id}} \right)} + {s^{2}\omega_{e}^{2}L_{d}L_{q}}} & \frac{{\left( {{L_{d}s^{2}} + {\left( {R + K_{pd}} \right)s} + K_{id}} \right)\left( {{\left( {\overset{\sim}{R} + K_{pq}} \right)s} + K_{iq}} \right)} + {s^{2}{\overset{\sim}{\omega}}_{e}^{2}{\overset{\sim}{L}}_{d}{\overset{\sim}{L}}_{q}}}{{\left( {{L_{d}s^{2}} + {\left( {R + K_{pd}} \right)s} + K_{id}} \right)\left( {{L_{q}s^{2}} + {\left( {R + K_{pq}} \right)s} + K_{iq}} \right)} + {s^{2}\omega_{e}^{2}L_{d}L_{q}}}\end{bmatrix}}}$

Terms T_(dd)(s) and T_(qq) (s) are the direct current to currenttransfer functions, while T_(dq)(s) and T_(qd) (s)represent the crosscoupling between the two current loops. For a typical system, the directtransfer functions have extremely high bandwidth.

A block diagram depicting a typical current sensor fault loss of assistmitigation algorithm 400 is shown in FIG. 4. As described in more detailbelow, a current regulator selector 402 may be selectively enabled ordisabled by a logic input. The current regulator selector 402 may selecta mode of feedforward control depending on the fault condition.Algorithms described in more detail below may also be implemented tomodify the torque command during the detection of the current sensorfault to ensure smooth transition from feedback control to feedforwardcontrol mode.

For example, a first ramp waveform as shown in FIG. 5 may be implementedby the torque command modifier, as described in more detail below, todecrease the torque command during a time t_(ramp) immediately after afault detection. FIG. 5 illustrates a torque command change upondetection of a fault. When a fault is detected, the torque commandmodifier may implement the ramp waveform shown in FIG. 5 by reducing thetorque command over a time period t_(ramp). After the time periodt_(ramp), a modified torque command is reduced by a scale factor k to amagnitude of the product of scale factor k and T_(org). The modifiedtorque command may be output by the torque command modifier as describedin more detail below.

A second torque ramp return waveform 600 is illustrated in FIG. 6. Insome cases, the torque ripple caused by the offset error may exceedrequirements related to maximum steering effort, so the torque commandis set to zero immediately after the current sensor fault is detected.Accordingly, after the torque command is set to a zero value, the torquecommand is increased over a time period t_(ramp). The torque command mayreturn to a steady-state value after time period t_(ramp). The modifiedtorque command may be a function of the scale factor k and may bereduced by this scale factor in a steady state condition. The modifiedtorque command may be reduced by a scale factor k as shown in FIG. 6.

Although two specific embodiments of torque ramp return waveforms areshown, the torque command modifier may be configured to implement anynumber of ramp return waveforms, and the subject application is notlimited to the waveforms shown in FIGS. 5 and 6. Further, otheralgorithms may be added to the torque command to avoid any disturbancethe driver may feel during the transition period.

FIG. 7 illustrates an open loop feedback current control block diagram700 where the feedback loop is opened and only static feedforwardcompensation is employed. The motor control current loop block diagramcan be simplified as shown. For this case, the direct transfer functionsbecome:

$\begin{matrix}{{T_{dd}(s)} = \frac{{L_{q}\overset{\sim}{R}s} + {R\overset{\sim}{R}} + {\omega_{e}{\overset{\sim}{\omega}}_{e}L_{d}L_{q}}}{{L_{d}L_{q}s^{2}} + {{R\left( {L_{d} + L_{q}} \right)}s} + R^{2} + {\omega_{e}^{2}L_{d}L_{q}}}} & \left( {{Equation}\mspace{14mu} 9} \right) \\{{T_{qq}(s)} = \frac{{L_{d}\overset{\sim}{R}s} + {R\overset{\sim}{R}} + {\omega_{e}{\overset{\sim}{\omega}}_{e}L_{q}L_{d}}}{{L_{d}L_{q}s^{2}} + {{R\left( {L_{d} + L_{q}} \right)}s} + R^{2} + {\omega_{e}^{2}L_{d}L_{q}}}} & \left( {{Equation}\mspace{14mu} 10} \right)\end{matrix}$

FIG. 8 represents a comparison plot 800 of a comparison of the frequencyresponses of the q-axis direct transfer functions at an operating speedof ω_(m)=200 rad/s. As shown in FIG. 8, static compensation providesundesirable frequency responses in terms of both magnitude and phase, ascompared to the feedback compensation.

To compensate for undesirable frequency responses, a stabilitycompensator of the steering control system may be changed at the timethe fault occurs. However, a stability compensator would have to betuned for the motor control loop bandwidth with the static feedforwardcontrol configuration. Further, since the stability compensator is anotch filter with various states, when the switching occurs, all thestate variables will get re-initialized to zero, causing a lag inresponse time. Additionally, a modified torque component may be requiredduring the transition.

FIG. 9A shows a steering control system 900A with one type of currentloop compensator 925A design. The configuration shown in FIG. 9A maycontrol the PMSM motor electrical plant of the motor 18 by generating anoutput current from an input voltage command. Specifically, FIG. 9Aincludes a static feedforward compensation module 922A and a currentregulation module 923A.

FIG. 9A also includes a steering control module 912A with a stabilitycompensator module 913A, a torque modifier module 914A, a stabilitycompensator selector module 915A and a current reference generatormodule 916A. A feedforward selection module 918A changes the mode of thecurrent regulator module 923A in the event a current sensor failure isdetected. The feedforward selection module 918A selects a mode of thecurrent regulator module 923A for the electric motor of the system,which is sent to the PMSM motor electrical plant of the motor 18.

The system may further include a current sensor fault detector module920A that detects an operational state of a current sensor (not shown).The current sensor fault detector module 920A may send an enable commandto the stability compensator selector module 915A, the torque modifiermodule 914A, and the feedforward selection module 918A.

In response to the detection of a current sensor fault by current sensorfault detector module 920A, the stability compensator selector module915A may implement a loss of assist mode in the steering system byselecting a loss of assist mode output from the stability compensatormodule 913A, and therefore generate a compensated torque command that issent to the torque command modifier module 913A. The selection of theloss of assist mode output changes a function provided by the stabilitycompensator module 913A upon the detection of the current sensor fault.The stability compensator module 913A may be tuned as a function of themotor control bandwidth, while the static feedforward controlconfiguration implemented in the current loop compensator 925A may notchange when the feedforward selection module 918A receives the enablecommand from the current sensor fault detector module 920A.

The stability compensator module 913A is, in some embodiments, a notchfilter that can be programmed with a plurality of states. During thechange of the function of the stability compensator module 913A ascontrolled by the stability compensator selector module 915A, statevariables of the stability compensator module 913A may be re-initializedto zero values, and over time, transition to values that represent theactual state of the steering system 900A.

Specifically, the torque modifier module 914A may implement the firstand second ramp waveforms as shown in FIG. 5 and FIG. 6, respectively,to assist with mitigation of any disruptions caused by the currentsensor fault.

The torque modifier module 914A may generate a modified torque commandin response to a current sensor fault. A magnitude of the modifiedtorque command may change over time and be consistent with the waveformsshown in FIG. 5 and FIG. 6. As emphasized above, the torque modifiermodule 914A is not limited to the implementation of the waveforms shownin FIGS. 5 and 6.

Turning to FIG. 9, this figure includes a steering control module 912, atorque modifier module 914, and a current reference generator module916. FIG. 9 further includes a feedforward selection module 918 thatenables a dynamic feedforward compensation in the event of a detectionof a sensor failure. The feedforward selection module 918 modifies thetorque command sent to the electric motor of the system, which isrepresented by the PMSM motor electrical plant of the motor 18.

The system may further include a current sensor fault detector module920 that detects an operational state of a current sensor (not shown).The current sensor fault detector module 920 may send an enable commandto the torque modifier module 914 and to the feedforward selectionmodule 918.

The torque modifier module 914 may implement the first and second rampwaveforms as shown in FIG. 5 and FIG. 6, respectively, to assist withmitigation of any disruptions caused by the current sensor fault. Thetorque modifier module 914 may generate a modified torque command inresponse to a current sensor fault. A magnitude of the modified torquecommand may change over time and be consistent with the waveforms shownin FIG. 5 and FIG. 6. As emphasized above, the torque modifier module914 is not limited to the implementation of the waveforms shown in FIGS.5 and 6.

The feedforward selection module 918 may select a dynamic feedforwardcompensation mode that processes a motor current command. The motorcurrent command may be generated by the current reference generator 916in response to the current reference generator 916 receiving the torquecommand modifier. The processing of the motor current command may changethe voltage commands sent to the electric motor in response to thecurrent sensor fault detection.

The dynamic feedforward compensation algorithm applied by thefeedforward selection module 918 may be performed by the dynamicfeedforward compensator module 922. The dynamic feedforward compensatormodule 922 may use a derivative transfer function implemented by aderivative estimation submodule (not shown). The dynamic feedforwardcompensator module 922 may modify a frequency response of a motorcontrol loop of the power steering system. Ideally, the derivativetransfer function is a true derivative that may be denoted by Laplacetransform variable s, however in some embodiments, the transfer functionmay be represented by an approximation of the derivative, {tilde over(s)}, as follows:

$\begin{matrix}{\overset{\sim}{s} = \frac{s}{\left( {{\tau \; s} + 1} \right)^{n}}} & \left( {{Equation}\mspace{14mu} 12} \right)\end{matrix}$

The derivative estimation submodule may be a high pass filter in someembodiments, but in other embodiments the derivative estimationsubmodule may be a discrete time derivative filter with specificmagnitude and phase characteristics.

In should be appreciated that although the static feedforward module 922is shown in FIG. 9, the static feedforward module 922 is not essentialfor operation of the system of FIG. 9 upon implementation of the dynamicfeedforward algorithm.

The stability compensator of the steering control module 912 is, in someembodiments, a notch filter that can be programmed with a plurality ofstates. During the change of the function of the stability compensatoras controlled by the stability compensator selector module, statevariables of the stability compensator may be re-initialized to zerovalues, and over time, transition to values that represent the actualstate of the steering system.

FIG. 10 includes a steering control module 1012 with a stabilitycompensator module 1013, a torque modifier module 1014, a stabilitycompensator selector module 1015 and a current reference generatormodule 1016. A feedforward selection module 1008 changes the mode of thecurrent regulator module 1023 in the event a current sensor failure isdetected. The feedforward selection module 1008 selects a mode of thecurrent regulator module 1023 for the electric motor of the system,which is sent to the PMSM motor electrical plant of the motor 18.

Specifically, the torque modifier module 1014 may implement the firstand second ramp waveforms as shown in FIG. 5 and FIG. 6, respectively,to assist with mitigation of any disruptions caused by the currentsensor fault.

Similar to the description provided in FIG. 9A, in response to thedetection of a current sensor fault by current sensor fault detectormodule 1020, the stability compensator selector module 1015 mayimplement a loss of assist mode in the steering system by selecting aloss of assist mode output from the stability compensator module 1013.The selection of the loss of assist mode output changes a functionprovided by the stability compensator module 1013 upon the detection ofthe current sensor fault.

In addition, a feedforward selection module 1008 enables a dynamicfeedforward compensation in the event of a detection of a sensorfailure. The feedforward selection module 1008 modifies the torquecommand sent to the electric motor of the system, which is representedby the PMSM motor electrical plant of the steering system mechanicalplant 1018.

FIG. 11 is a simplified block diagram for the motor control current loopunder fault condition with dynamic feedforward compensation employed.The derivative term is shown as s. The derivative compensator includedin the derivative estimation module is an approximation of a truederivative. In general, many different types of derivative filterdesigns may be used, from simple high pass filters to more sophisticateddiscrete time derivative filters with specific magnitude and phasecharacteristics, depending on the application.

For FIG. 11, the direct transfer functions become:

$\begin{matrix}{{T_{dd}(s)} = \frac{{L_{q}{\overset{\sim}{L}}_{d}s\overset{\sim}{s}L_{q}\overset{\sim}{R}s} + {{\overset{\sim}{L}}_{d}R\overset{\sim}{s}} + {\omega_{e}{\overset{\sim}{\omega}}_{e}{\overset{\sim}{L}}_{d}L_{q}}}{{L_{d}L_{q}s^{2}} + {{R\left( {L_{d} + L_{q}} \right)}s} + R^{2} + {\omega_{e}^{2}L_{d}L_{q}}}} & \left( {{Equation}\mspace{14mu} 13} \right) \\{{T_{qq}(s)} = \frac{{L_{d}{\overset{\sim}{L}}_{q}s\overset{\sim}{s}L_{d}\overset{\sim}{R}s} + {{\overset{\sim}{L}}_{q}R\overset{\sim}{s}} + {\omega_{e}{\overset{\sim}{\omega}}_{e}{\overset{\sim}{L}}_{q}L_{d}}}{{L_{d}L_{q}s^{2}} + {{R\left( {L_{d} + L_{q}} \right)}s} + R^{2} + {\omega_{e}^{2}L_{d}L_{q}}}} & \left( {{Equation}\mspace{14mu} 14} \right)\end{matrix}$

It can be appreciated from equations 13 and 14 that if the derivativefilter were ideal, both the transfer functions would simply becomeunity. The derivative filter is contained within the derivativeestimation module 1110 in FIG. 11.

If the current loop has a different configuration, and does not have acomplete feedforward compensator during normal operation, then the fulldynamic feedforward compensation terms can be calculated continuously,but applied only during the fault condition.

As used above, the term “module” or “sub-module” refers to anapplication specific integrated circuit (ASIC), an electronic circuit, aprocessor (shared, dedicated, or group) and memory that executes one ormore software or firmware programs, a combinational logic circuit,and/or other suitable components that provide the describedfunctionality. When implemented in software, a module or a sub-modulecan be embodied in memory as a non-transitory machine-readable storagemedium readable by a processing circuit and storing instructions forexecution by the processing circuit for performing a method. Moreover,the modules and sub-modules shown in the above Figures may be combinedand/or further partitioned.

While the invention has been described in detail in connection with onlya limited number of embodiments, it should be readily understood thatthe invention is not limited to such disclosed embodiments. Rather, theinvention can be modified to incorporate any number of variations,alterations, substitutions or equivalent arrangements not heretoforedescribed, but which are commensurate with the spirit and scope of theinvention. Additionally, while various embodiments of the invention havebeen described, it is to be understood that aspects of the invention mayinclude only some of the described embodiments. Accordingly, theinvention is not to be seen as limited by the foregoing description.

Having thus described the invention, it is claimed:
 1. A power steering system comprising: a torque modifier module that generates a modified torque command in response to a current sensor fault, a magnitude of the modified torque command changes over a time period; and a feedforward selection module that applies a dynamic feedforward compensation to a motor current command, thereby generating a motor voltage that is applied to a motor of the power steering system, the dynamic feedforward compensation modifies a frequency response of the power steering system, the motor current command is based on the modified torque command.
 2. The power steering system of claim 1, further comprising a stability compensator selector module that selects a stability compensator of a steering torque control loop of the power steering system when a current sensor fault is detected, the stability compensator selector module generates a compensated torque command sent to the torque modifier module.
 3. The power steering system of claim 2, the modified torque command is based at least in part on the compensated torque command.
 4. The power steering system of claim 1, the modified torque command is reduced to a magnitude of zero when the current sensor fault is detected, the modified torque command is increased from a magnitude of zero over a time period.
 5. The power steering system of claim 4, the modified torque command has a steady state value defined by a scale factor applied to the modified torque command during the time period.
 6. The power steering system of claim 2, the dynamic feedforward compensation is based on a transfer function defined by an approximation of a true derivative.
 7. The power steering system of claim 6, the approximation of the true derivative is represented by $\overset{\sim}{s} = {\frac{s}{\left( {{\tau \; s} + 1} \right)^{n}}.}$
 8. A power steering system comprising: a stability compensator selector module that selects a stability compensator of a steering torque control loop of the power steering system when a current sensor fault is detected, the stability compensator generates a compensated torque command; and a torque modifier module that generates a modified torque command from the compensated torque command in response to a current sensor fault, a magnitude of the modified torque command changes over a time period, a motor voltage that is applied to a motor of the power steering system is based on the modified torque command.
 9. The power steering system of claim 8, further comprising a feedforward selection module that applies a dynamic feedforward compensation to a motor current command, thereby generating the motor voltage that is applied to a motor of the power steering system, the dynamic feedforward compensation modifies a frequency response of the power steering system, the motor current command is based on the modified torque command.
 10. The power steering system of claim 8, the modified torque command is increased from a magnitude of zero when the current sensor fault is detected.
 11. The power steering system of claim 10, the modified torque command has a steady state value defined by a scale factor applied to the modified torque command during a time period.
 12. The power steering system of claim 9, the dynamic feedforward compensation is based on a transfer function defined by an approximation of a true derivative.
 14. The power steering system of claim 12, the approximation of the true derivative is represented by $\overset{\sim}{s} = {\frac{s}{\left( {{\tau \; s} + 1} \right)^{n}}.}$
 15. A method for controlling a power steering system comprising: generating a modified torque command in response to a current sensor fault, a magnitude of the modified torque command changes over a time period; and applying a dynamic feedforward compensation to a motor current command, thereby generating a motor voltage that is applied to a motor of the power steering system, the dynamic feedforward compensation modifies a frequency response of the power steering system, the motor current command is based on the modified torque command.
 16. The method of claim 15, further comprising selecting a stability compensator of a steering torque control loop of the power steering system when a current sensor fault is detected, the stability compensator selector module generates a compensated torque command.
 17. The method of claim 15, the modified torque command is increased from a magnitude of zero when the current sensor fault is detected.
 18. The method of claim 17, the modified torque command has a steady state value defined by a scale factor applied to the modified torque command during a time period.
 19. The method of claim 15, the dynamic feedforward compensation is based on a transfer function defined by an approximation of a true derivative.
 20. The method of claim 19, the approximation of the true derivative is represented by $\overset{\sim}{s} = {\frac{s}{\left( {{\tau \; s} + 1} \right)^{n}}.}$ 